For this entire century, the factory has been telling people that they never have to change the coolant in their cars. Maybe they figured that most of them would spring a leak and get changed anyway. Long ago, H&B Karmakanix sorted out some cooling systems that do not tend to leak, and the result is that the old coolant is now clogging up their heater cores, and an occasional radiator. And we simply did not have this issue last century when coolant got changed on a regular basis. When coolant gets too old, deposits start to form, eventually resembling an eggshell coating inside the system. And if someone adds some sort of StopLeak or well water with a high mineral content, the deposits are more like concrete.